Metal foams, which are lightweight and high-strength porous metallic structures, are gaining increasing interest in numerous industries, such as the aerospace and automotive industries. In particular, the introduction of components formed from metal foam materials into aerospace or automotive structures may lead to improvements in fuel efficiency, while providing other beneficial properties such as vibration dampening, erosion resistance, and enhanced mechanical strength and overall performance. Moreover, metal foams may have high temperatures resistances and, therefore, may provide thermal protection properties for a range of applications as well.
Nanocellular metal foams are a sub-class of metal foams which have pore sizes in the nanoscale or submicron range. Open-celled nanocellular metal foams, which have open and gas-filled pores, may appear as a network of interconnected ligaments that form the solid, metallic portion of the metal foam. The diameters of the ligaments (as measured by the width of the ligament at its narrowest part) may be correlated with the strength-to-weight ratio of the metal foam. In particular, it has been predicted that the strength of a nanocellular metal foam may approach the strength of an identically-sized solid metal part as its ligament diameters decrease, while at only a fraction of the weight of the solid metal part. For at least this reason, nanocellular metal foams having high integrity ligaments with diameters on the nanoscale or submicron scale may be a desirable target for many engineers. Despite the benefits that such lightweight and high-strength materials may provide for numerous applications, it currently remains a challenge to fabricate metal foams with ligament diameters below one micron.
Current methods for producing stochastic metal foams may use powder metallurgy in which a metal powder may be mixed with a foaming agent and compacted to a dense structure. The metal and foaming agent mixture may then be heated to cause the foaming agent to release gas and expand the metal material, causing it to form a porous structure. Such methods for producing metal foams have been described, for example, in U.S. Pat. Nos. 6,444,007 and in 2,751,289. In addition, electroplating may also be used to produce metal foams. While effective, the existing metal foam fabrication methods may fail to provide metal foams having ligament diameters below one micron. Furthermore, these fabrication methods may offer limited control over the ligament diameters of the metal foams and their corresponding mechanical properties. Clearly, there is a need for fabrication methods capable of producing metal foams with ligament diameters on the submicron scale.